Multiple sighting methods for aiming firearms have been utilized for both commercial and military applications. The most basic, the iron sight, requires the shooter to align the rear sight, front sight, and the target while switching the eye's point of focus between the rear and front sight pattern and the target. The iron sight method, while reliable, can be difficult to learn and has multiple drawbacks including the rear and front sight obstructing the view of the target and complications arising from sub-optimal lighting conditions. Telescopic sights, while accurate, are not suited for situations where the target is moving, close quarter conditions, or if the shooter is mobile. Laser designator sights, where a laser beam illuminates the target can be difficult to use in certain lighting conditions and are unacceptable for military or self-defense applications where the shooters location must not be revealed. Red dot sighting systems—Reflex-, such as those provided by AimPoint® solve many of the above difficulties, but are only able to project a single dot reticule pattern due to off-axis aberrations.
Holographic sighting systems, such as described in U.S. Pat. No. 6,490,060 issued to Tai et al., have become the preferred solution for shooters requiring a high degree of accuracy with fast target acquisition. Specifically, the Holographic Weapon Sight (HWS) manufactured by L3-Communications EOTech, Inc. was the first system to make use of holographic technology in order to project a full reticule pattern at or near the target plane creating a low parallax sighting solution that can be deployed quickly and in any lighting condition. EOTech holographic weapon sight (HWS) systems were also the first to make use of wavelength dispersion matching technology that allow laser diode illumination to be used over extended temperature ranges.
However, currently available HWS systems suffer from drawbacks related to their design and technique of manufacturing. Current HWS products utilize an internal mechanism to adjust reticule position, which alters the achromatic geometry of the system when aiming adjustments are made, thereby compromising the ability of the wavelength dispersion compensation elements to perform as intended. The result is that these sighting systems will perform differently when used in temperatures other than that of which they were initially aligned. In addition to compromised wavelength dispersion matching, current HWS products also suffer from reticule pattern distortion as a result of the mechanical stresses created from the internal adjustment mechanism. The wavelength stabilizing holographic optical element is bonded to a plastic flexible element, creating a system that is susceptible to wave front aberrations brought about by changes in temperature to the hologram and mounting substrate. The resulting distortions can be observed as a “smudging” or increase in size of the dot element used for aiming. In addition, current HWS products use a standard o-ring and compressed flat rubber gasket system to isolate the holographic optical components from the outside environment. The effectiveness and reliability of an o-ring and compressed flat rubber gasket sealed HWS system is highly dependent on the repeatability of the manufacturing process. Any variability induced by workmanship and manufacturing methods affects the integrity and effectiveness of the seal over time. The flat rubber and o-ring sealing system is often compromised by the stresses caused from changing ambient temperature and pressure conditions, resulting in continued cycling of the housing cavity pressure, which tends to make the flat gasket method of sealing failure prone. As a consequence, systems sealed in this manner tend to leak, allowing moisture to permeate the holographic elements resulting in fading of the reticule image.
As a result, the current HWS systems require higher intensities of laser diode power in order to maintain a desired reticule brightness and have average battery life spans far less than competing red-dot products.
Most current HWS products use an off axis reflection collimating element to prepare the light incident on the first holographic element. These reflection collimating elements require the precision removal of an off center section from a lens to be coated with a reflective element prior to their use in the HWS. However, inconsistencies in the production of this off center section introduce aberrations and increases production costs.
Finally, current designs for HWS systems lack modularity, and minor changes to mounting systems, optical path, reticule patterns, and battery type require a complete re-design and re-tooling in order to implement the changes.
Thus, there exists a need for a modular lightweight holographic sighting system that is stable over a wide temperature range, maintains a battery life that is comparable to competing red-dot products, has a large, high aspect viewing window, occupies the minimum amount of rail space on a hand held weapon, and can be produced inexpensively and accurately in high volume.